public class YourAdapter extends XmlAdapter<List<Variable>,Map<String,String>>{
public Map<String,String> unmarshall(List<Variable> arg0){
//Logic to convert a List<Variable> into a Map<String,String> and return it
}
public List<Variable> marshall(Map<String,String> arg0){
//Logic to convert a Map<String,String> into a List<Variable> and return it
}
}